Top drives are used to suspend and rotate a string of drill pipe and/or casing in drilling applications. The top drive is supported by a drilling line wrapped on a set of sheaves and connected to drawworks at one extremity. The top drive supports the drill string via a thrust bearing. Mud may be pumped into the drill string via a swivel. Furthermore, the top drive generally includes one or more motors (electric or hydraulic) which generate(s) the rotation of the drill string. The reaction torque applied to the top drive may be transmitted to the mast via a set of rollers attached to the top-drive chassis.
Various measurements may be used to manage the drilling process, including those that involve the top drive. Hook load and hook elevation above the rig floor are two examples of such measurements. These measurements may be employed to calculate drilling parameters such as weight on bit (WOB), rate of penetration (ROP), and depth. A variety of other types of measurements are used to calculate these and other drilling parameters.
Automated control of drilling operations may be employed in connection with the top drive. Such automation may improve the rate of penetration, reduce stick and slip, and reduce damage to the bottom hole assembly (BHA). There may be three main parameters that are available for the driller to affect the drilling operation, namely: revolutions per minute (RPM), block velocity, and flow rate. The RPM may be controlled by adjusting the top drive, the block velocity may be controlled through the drawworks, and flow rate may be controlled through the pump. Automated drilling may depend on controlling the drawworks, for example, to maintain a constant WOB or ROP. However, the drill line may be a highly-damped system, making fine control of the block velocity difficult to accomplish.